Process for recovering solid calcium fluoride containing product and colloidal silica solution from a weak aqueous fluosilicic acid solution



Feb. 5, 1957 G. H. GLoss 2,780,523

PROCESS FOR RECOVERING SOLID CALCIUM FLUORIDE CONTAINING l PRODUCT AND COLLOIDAI.. SILICA SOLUTION FROM A WEAK' AQUEOUS FLUOSI-LICIC ACID SOLUTION Filed March 22, 1954 moou Y wN/.Nou

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. H m40 4 OI United States ,Patent O PROCESS FOR RECOVERING SOLID CALCIUM FLUORIDE CONTAINING PRODUCT AND COL- LOIDAL SILICA SOLUTION FROM A WEAK AQUEOUS FLUOSILICIC ACID SOLUTION Gunter H. Gloss, Libertyville, Ill., assignor to International Minerals & Chemical Corporation, a corporation of New York Application March 22, 1954, Serial No. 417,723

Claims. (Cl. 23-88) This invention relates to the production of iluoride` chemicals. More particularly it relates to a process for the manufacture of calcium fluoride and silica gel from waste gases.

Large quantities of waste gases containing hydrotluoric acid and/or silicon tetrafluoride are evolved in many industrial operations and present a serious disposal problem. Atmospheric pollution is due not only to the presence of hydrofluoric acid, but also to the presence of silicon tetrafluoride, which in contact with moisture readily hydrolyzes to yield quantities of fluosilicic acid.

Manufacture of fluorides commercially has in general been limited to continuously neutralizing basic solutions with aqueous or anhydrous hydrofluoric acid. Calcium iluoride has been produced by absorbing hydrolluoric acid on dry oolitic or pesolitic limestones, or by neutralizing aqueous hydrouoric acid with precipitated calcium carbonate. These methods all involve the use of relatively pure hydrofluoric acid for a neutralization reaction.

Recovery of uorine from waste gases has been carried out by washing out the water soluble constituents of gases in an absorber. Such absorber treatments produce only a dilute solution of fluosilicic acid and such dilute solutions have not in general been adaptable to the manufacture of pure fluoride chemicals. Aqueous acid solutions prepared in an absorber have been treated with sodium chloride or calcium chloride to precipitate either sodium fluosilicate or calcium uosilicate. When the aqueous acid solution has been treated with calcium carbonate as a substitute for calcium chloride, the product generally has been a mixed precipitate of calcium uoride, calcium fluosilicate and silica. l

In a copending application by Charles A. Butt, Sen'al No. 417,965 led of even date herewith, there is shown a process for neutralizing liuosilicic acid solutions with calcium carbonate and separately recovering calcium tluoride and silica gel. The calcium fluoride product recovered in this system is not pure, however, and generally carries with it about 4% to about 7% by weight of silica.

A principal dimculty in carrying out the processes above described has been the precipitation of calcium fluoride in a manner which permits recovery of economic quantities of pure calcium uoride meeting the specifications for commercial iiuorspars of less than 1.5% SiOz. In addition to the above shortcoming, processes which fail to precipitate the calcium fluoride and silica separately in general bring down a gelatinous or flocculent precipitate which presents ltering problems.

It is a primary object of this invention, therefore, to overcome the ditiiculties and shortcomings of processes heretofore in use.

lt is another object of this invention to precipitate calcium uoride in a manner permitting recovery of a calcium uoride product containing less than about 0.5% silica on a dry basis.

It is still a further object of this invention to recover 2,780,523 Patented Feb. 5, 1957 ice line earth compounds capable of forming uorides of relatively low solubility in aqueous media. In the first stage, calcium carbonate is added in amounts up to approximately 85% of the stoichiometric equivalent requirements for complete reaction between fluosilicic acid and calcium carbonate. After removal of the precipitated solids the remainder of the calcium carbonate necessary for complete reaction is added to the filtrate. Upon removal of the second precipitate, the substantially i uorine-free solution is treated to remove free water and give a pure silica gel product.

In more detail, if the uorine-containing gases are a by-product of the manufacture of phosphoric acid or phosphate fertilizers, the den gases are passed through an absorber wherethe fluorine-bearing constituents of the gases are scrubbed out by an aqueous medium such as water or aqueous uosilicic acid solution. In order to remove phosphate dust the gases can first be passed through a scrubber where they are contacted with saturated fluosilicic acid, sulfuric acid, or phosphoric acid, as shown in copending application Serial No. 398,866, Gunter H; Gloss and .lohn H. Gross, inventors, led December 17, 1953, entitled, Production of Fluorine Compounds.

The aqueous acidic euent solution from the absorber is controlled to havea fluosilicic acid concentration of between about 21/2 HzSiFs and about 3.8% HzSil-"s` with concentrations-in the range of about 3% to about 3.5% HzSiFs preferred.

If the acid concentration exceeds about 4% in the instant process the SiOz content of the CaFz product increases rapidly and the ilterability of the solution drops off rapidly. 'Before addition of calcium carbonate, the liuosilicic acid solution from the scrubber is treated to remove any -solids or precipitated silica as by filtering, settling and decantation or an equivalent operation.

Aqueous acidic solution is next reacted in a rst precipitation stage with nely divided calcium carbonate in amounts giving a mole ratio not exceeding 85% of the stoichiometric amount for complete reaction, i. e., 85% of the amount necessary to give the ratio l mole HzSiFs to 3 moles CaCOs. ln general the first addition may vary 40% to 80% of the stoichiometric amount, depending upon the calcium fluoride recoveiy and purity desired. The purity of the product obtained after addition when dried, at 110 C. will have a calcium luoride content of 92-93%. By calcination, the residual water and carbon dioxide can be driven oi and the resultant product has a purity of 97-99% calcium fluoride.

The time of addition of calcium carbonate plus holding time may vary from about 30 minutes to about 2 hours duration. Either addition of amounts of CaCO3 in excess of or longer reaction times are to be avoided, since both increase the amount of co-precipitated hydroussilica in the calcium fluoride product. The length of the reaction time in this rst stage varies depending upon the yduration of the reaction time permitted in the second stage as discussed subsequently.

In general the lower the temperature of the reacting mixture, the higher is the purity of calcium liuoride, other variables being held constant. A reaction temperature 3. between about 35 F. and about 100 F. or higher can be used, but temperatures in the range of about 60 F. to about 90 F. are preferred because while calcium fluoride purity goes down as the reaction temperature is increased, the rate of change of purity is small by comparison with the change in filtration rate.

Solution from the first stage reaction is treated with the balance of the stoichiometric amount of calcium carbonate or preferably with an amount giving a slight excess of calcium carbonate such that a pH in the range of about 7 to about 7.3 is obtained in the final solution.

Temperatures are maintained at approximately the same level in the second stage of reaction as in the first stage. Reaction time in this second stage, however, must be subject to closer control if filtration problems are to be avoided. The overall reaction time for both of the reaction stages should generally not exceed 3 hours and preferably not exceed a total of 2 hours. The longer the reaction time permitted in the first stage, the shorter is the allowable reaction time in the second stage.

The second stage reaction results in the precipitation of a mixture ofcalcium fluoride and calcium silicofluoride. After the removal of this mixed precipitate, the resultant filtrate is concentrated by suitable means such as low temperature evaporation and drying.

The process will be further explained by reference to the flow sheet illustrating but one embodiment of the invention, in which den gases from a source not shown are passed counter-current to liquid in an absorber indicated by the numeral 10. Aqueous medium plus absorbed gases are withdrawn from the absorber 10 in the form of aqueous acidic solution through conduit 11. A part of the liquid is returned through conduit l2 to the top of the absorber where it is mixed with fresh water from a'source not shown. The balance of the aqueous acidic solution is diverted through conduit 13 to filter station 14 Where precipitated silica and miscellaneous solid impurities are removed. The filtrate from filter station 14 is passed through conduit 15 to a reactor 16 where with agitation the solution is mixed with the first charge of calcium carbonate. Carbon dioxide that is evolved by the reaction is discharged from the reactor through vent 17. Slurry from the reactor is pumped through conduit 18 to a filter station 19 where a substantially pure calcium fluoride cake 20 is separated. Calcium fluoride cake 20 is conveyed from filter 19 to a dryer 21 by suitable means 22 such as a moving belt. Filtrate from filter station 19 is pumped through conduit 23 to the second reactor 24. In this reactor the balance of calcium carbonate is added and the carbon dioxide evolved is discharged through vent 25. Slurry from reactor 24 is delivered through conduit. 26 to filter station 27. Filtercake from station 27 is delivered by a suitable conveyor 28 to a storage station for further processing. The filtrate from filter station 27 is pumped through conduit 29 to storage-vessel 30 where the colloidal solution is allowed to geL The gel is delivered by means of conveyor 31 to drying facilities 32 from which is obtained the dry silica gel product.

The following example is given by way of further explanation and without any intention of limiting the invention thereto.

Example I Florida phosphate rock and sulfuric acid mixed in proportions to produce ordinary superphosphate evolved den gases which when passed through an absorber countercurrent to fresh water added to the circuit produced an efiiuent liquid totaling approximately 5000 parts by weight and analyzing approximately 3.5% HzSiFe. Approximately 5000 parts by weight of this solution when treated with approximately 290 parts by weight of calcium carbonate, addition being made over a period of l hour, evolved approximately 127 parts by weight of carbon dioxide. The slurry formed by this reaction contained approximately 4.35% by weight of suspended CaFz, while Cal the aqueous phase contained 0.68% of residual HzSiFs and 1.1% of colloidal silica. The slurry from this reaction vessel was filtered and 225 parts by weight of cake recovered which upon drying at 300 F. analyzed 92% CaFz and 0.52% SiOz. The filtrate from this first filter operation when treated in reactor No. 2 with 156 parts by weight of calcium carbonate evolves approximately 32 parts by Weight of carbon dioxide. Filtration of the slurry from the reactor No. 2 yields approximately 59 parts by dry weight of cake consisting predominantly of calcium fluoride and calcium silicofluoride. Filtrate obtained in this second filtration operation analyzes approximately 1% to 2% SiOz. After gelation and drying at 550 C. this product had the following composition:

Percent SiOz 88.05 F 0.00 Ca 0.35 Co2 0.42 Combined H2O 11.15

Having thus described my invention, what I claim is:

l. A process for the production of fluoriue-containing compounds which comprises reacting an aqueous fluosilicic acid solution containing less than 4% HzSiFs with an amount of finely divided calcium carbonate which is less than about of the stoichiometric equivalent required for neutralization of the HzSiFs content, recovering a calcium fluoride solid product from a first solution containing colloidal silica, neutralizing said first solution with an added quantity of calcium carbonate, which will produce a second solution having a pH in the range of between about 7 and about 7.3, and separately recovering a second calcium fluoride-containing solid product and a second solution containing colloidal silica.

2. A process for recovering fluorides from gases containing the same, which comprises scrubbing said gases with aqueous medium to recover an acidic solution containing between about 2.5% and about 3.8% HzSiFe, r:- acting said solution with an amount of finely divided calcium carbonate which is less than about 85% of the stoichiometric equivalent required for neutralization of the HzSiFs content, recovering a calcium fluoride solid product from a first solution containing colloidal silica, neutralizing said first solution with an added quantity of calcium carbonate and separately recovering n second calcium fluoride-containing soli-:l product and a second solution containing colloidal silica.

3. A process as in claim 2 in which the second solution containing colloidal silica is concentrated and silica gel is recovered therefrom.

4. A process for recovering fluorides from gases containing the same which comprises scrubbing gases with aqueous medium to recover an acidic aqueous solution containing between about 3 and about 3.5% HQSiFs, reacting said aqueous acidic solution with an amount of finely divided calcium carbonate which is less than about 85% of the stoichiometric equivalent required for ncutralization of the HzSiFG content, recovering a calcium fluoride solid product from a first solution containing colloidal silica, neutralizing said first solution with an added quantity of calcium carbonate, separately recovering a second calcium fluoride-containing solid product and a second solution containing colloidal silica, concentrating the second solution and recovering silica gel therefrom.

5. A process for the production of ilumine-containing compounds which comprises reacting an aqueous fluosilicic acid solution containing less than 4% H2SiFs at a emperature between about 35 F. and about 100 F. 'with an amount of finely divided calcium carbonate which constitutes between about 40 and about 80% of the stoichiometric equivalent required for neutralization of the HzSiFs content, recovering a calcium fluoride solid product from a first solution containing colloidal silica,

neutralizing said first solution with an added quantity of calcium carbonate, and separately recovering a second calcium uoride-containing solid product and a second solution containing colloidal silica.

6. A process for the production of ilumine-containing compounds which comprises reacting an aqueous fluosilicc acid solution containing less than 4% HaSiFs at a temperature in the range between about 60 F. and about 90 F. with an amount of nely divided calcium car bonate which is less than about 85% of the stoichiometric equivalent required for the neutral-ization of the HzSiFs content, recovering a calcium uoride solid product from a first solution containing colloidal silica, neutralizing said rst solution with an added quantity of calcium carbonate suflicient to produce a second solution having a pH in the range between about 7 and about 7.3, and separately recovering a second calcium iluoridecontaining solid product and a second solution containing colloidal silica.

7. A process as in claim 6 wherein the second solution containing colloidal silica is concentrated and silica gel is recovered therefrom.

8. A process for the production of ilumine-containing compounds which comprises reacting an aqueous fluosilicie acid solution containing between about 2.5% and about 3.8% HzSiFs over a period in the range of between about 30 minutes and about two hours with an amount of finely divided calcium carbonate which is less than about 85% of the stoichiometric equivalent required for neutralization of the HzSiFs, recovering a calcium uoride solid product from a lirst solution containing colloidal silica, neutralizing said rst solution with calcium carbonate in excess of the stoichiometric equivalent required for completing neutralization of the HzSiFs over a period of time such that the total of reaction times for the rst neutralization and the second neutralization is less than 3 hours, separately recovering a second calcium uoride-containing solid product and a second solution containing colloidal silica.

9. A process for the production of uorinecontainiug compounds which comprises scrubbing den gases pro duced by the reaction of phosphate rock; with sulfuric acid in the manufacture of superphosphate with aqueous medium to produce an aqueous ellluent containing between about 2.5 and about 3.8% iluosilicic acid, reacting said solution with between about and about 80% of the stoichiometric equivalent required for neutralization of the HzSiFs, ltering the slurry to recover a calcium fluoride cake and a lirst solution containing colloidal silica, neutralizing the said rst solution with additional calcium carbonate in an amount suicient to produce a solution having a pH between about 7 and about 7.3, filtering the slurry and separately recovering a calcium fluoride-calcium silicotiuoride precipitate and a second ltrate containing colloidal silica.

10. A process as in claim 9 wherein the second filtrate is evaporated and silica gel recovered therefrom.

References Cited in the le of this patent UNITED STATES PATENTS 1,456,594 Howard May 29, 1923- 2,385,208 Jones Sept. 18, 1945 2,447,359 Oakley Aug. 17, 1948 2,573,704 Gilbert et al. Nov. 6, 1951 2,584,894 Maclntire Feb. 5, 1952 2,702,233 Mitchell et al. Feb. 15, 1955 OTHER REFERENCES Fluorine Control and Recovery, by D. D. Morris, B. P. Sutherland and C. H. Wright, Canadian Chem. and Metallurgy, August 1937, pages 271-273. 

1. A PROCESS FOR THE PRODUCTION OF FLUORINE-CONTAINING COMPOUNDS WHICH COMPRISES REACTING AN AQUEOUS FLUOSILICIC ACID SOLUTION CONTAINING LESS THAN 4% H2SIF5 WITH AN AMOUNT OF FINELY DIVIDED CALCIUM CARBONATE WHICH IS LESS THAN ABOUT 85% OF THE STOICHIOMETRIC EQUIVALENT REQUIRED FOR NEUTRALIZATION OF THE H2SIF5 CONTENT, RECOVERING A CALCIUM FLUORIDE SOLID PRODUCT FROM A FIRST SOLUTION CONTAINING COLLODIAL SILICA, NEUTRALIZING SAID FIRST SOLUTION WITH AN ADDED QUANTITY OF CALCIUM CARBONATE, WHICH WILL PRODUCE A SECOND SOLUTION HAVING A PH IN THE RANGE OF BETWEEN ABOUT 7 AND ABOUT 7.3, AND SEPARATELY RECOVERING A SECND CALCIUM FLUORIDE-CONTAINING SOLID PRODUCT AND A SECOND SOLUTION CONTAINING COLLODIAL SILICA. 